Ammonium sulfate nitrate (ASN), one of the first synthetic fertilizers, has been in continuous use for nearly 100 years providing the important primary and secondary nutrients, nitrogen and sulfur. Nitrogen is provided in part through the nitrate ion, desirable because it is readily absorbed by many plants and promotes early growth.
ASN fertilizers are generally produced by granulation of ammonium sulfate (AS) solids with ammonium nitrate (AN) liquors. Alternatively, ASN fertilizers may be processed as a melt-slurry from either the constituent fertilizers (ammonium nitrate and ammonium sulfate) or, alternatively, from the reaction products of sulfuric/nitric acids with anhydrous ammonia. There are several alternatives for converting the melt-slurry into solid fertilizer particles with the physical properties desired by various markets.
Prilling is a process in which a liquid melt is gently streamed from the top of a tall structure with counter-current cooling air. Surface tension divides the stream into individual droplets which solidify before reaching the tower bottom. An illustrative example of prilling may be found in U.S. Pat. No. 7,175,684, which is incorporated by reference herein.
Pastillation is similar to prilling in that the melt is converted to a droplet and then solidified. However, it differs from prilling in two distinct aspects. First, rather than relying on surface tension to size the droplets, the droplets are portioned mechanically thus achieving very high uniformity in size. Second, rather than dropping the droplet through cooling air, the droplet is applied to a water-cooled metal belt. The heat is removed through the belt and solidified particles fall off the belt at its end. An illustrative example of pastillation may be found in U.S. Pat. No. 7,985,393, which is incorporated by reference herein.
In one type of granulation process, a melt-slurry is sprayed onto a moving bed of granules. The melt-slurry both coats and agglomerates bed granules to increase in size. In some embodiments, the granules are discharged to a dryer drum which, if included, provides additional rolling time for the granules. The granules pass to a screening operation where the product cut is recovered and the under-sized and over-sized material is recycled back to the granulation drum.
It was previously disclosed in U.S. Pat. No. 6,689,181, which is incorporated herein by reference in its entirety, that ammonium sulfate and ammonium nitrate form double-salt compounds during production, including a 3:1 AN to AS double salt (known as a 3:1 double salt) and a 2:1 AN to AS double salt (known as a 2:1 double salt). The '181 patent recognized the benefit of the 2:1 double salt as being a more stable compound. Additional examples of methods of producing 2:1 double salts are provided in U.S. Patent Publication No. 2010/0047149, which is also incorporated by reference herein.